1. Field of the Invention
This invention relates generally to the art of depositing a coating on a substrate, especially a glass substrate, and relates more particularly to the art of depositing a coating from a powder coating composition.
2. Discussion of the Technology
Various methods of coating glass with metal or metal oxide films are well-known in the art. A technique for depositing a variety of metal oxide films onto a hot glass surface in a continuous float glass ribbon environment is described in U.S. Pat. No. 3,660,061 to Donley et al. A mixture of organometallic compounds in organic solution is sprayed onto a glass surface at a temperature high enough for thermal reaction of the organometals to form a metal oxide film. This technique produces durable metal oxide films having desirable aesthetic and solar energy control properties. Significant disadvantages are the health, safety and environmental effects of using large volumes of organic solvents.
These disadvantages may be abated by elimination of the organic solvent. A method for solventless chemical vapor deposition of coatings from vaporized powder coating reactants is described in U.S. Pat. No. 3,852,098 to Bloss et al. A powder coating reactant is dispersed into a hot stream of gas, vaporized, and conveyed to the surface to be coated, which is maintained at or above the temperature at which the coating reactant pyrolyzes to deposit a film. Although the disadvantages of a solvent system are avoided, vaporization of the coating reactant requires high temperatures, with the possibility of premature thermal reaction or decomposition of some coating reactants.
Another method of vapor deposition is described in U.S. Pat. No. 4,182,783 to Henery, wherein a solid particulate coating reactant is fluidized by introducing a volume of fluidizing gas through a mass of reactant. The fluidized mixture of coating reactant and gas is diluted with an additional volume of gas prior to delivery to the surface of the substrate to be coated. An apparatus for carrying out the technique of fluidizing a bed of solid particulate coating reactant is illustrated in U.S. Pat. No. 4,297,971 to the same inventor.
A method which avoids the health, safety and environmental problems of a solvent-based coating method, the high-temperature vaporization risks of a vapor deposition method, and the complexity of a fluidized bed powder coating method is disclosed in U.S. Pat. No. 4,325,988 to Wagner, which teaches a method and apparatus for dispersing powder coating reactants in a carrier gas stream and delivering the powder coating composition uniformly to the surface of a substrate to be coated. A powder coating reactant is obtained in very fine particle size, for example by microjet milling, and mixed with a carrier gas stream. Turbulence of the mixture is obtained by means of a baffle to maintain uniform distribution of the powder coating reactant in the carrier gas en route to the substrate to be coated. The powder coating reactant is delivered to the substrate through a nozzle positioned a short distance from the surface to be coated. The length of the nozzle is typically greater than its width, preferably substantially equal to the parallel dimension of the substrate, and is generally disposed perpendicular to the direction of relative motion between the slot and the substrate. The carrier gas is typically air supplied at a rate of 50 cubic feet per minute at a pressure of 75 pounds per square inch.